Cast iron



Patented Nov. 1 5,

CAST IRON Henton Morrogh, Handsworth, Birmingham,

England, assignor to British Cast Iron Research Association,Al'vechurch, Birmingham,

England No Drawing. Application April 28, 1948, Serial No. 23,859. InGreat Britain May 21. 1947 Claims.

This invention relates to improved cast irons of the corrosion-resistanttype containing more than 10% 'byweight of silicon.

For the purpose of making castings resistant to corrosion by strongchemical reagents, a special cast iron is frequently used whichcontains, as its most important constituent, a large proportion ofsilicon. The usual alloying elements, namely, manganese, carbon,phosphorus, sulphur, are always. present and other elements such asmolybdenum, copper, nickel, chromium, titanium and aluminium may bepresent either fortuitously or by design. For the purpose ofcorrosion-resistance the important element in these alloys is silicon,which may be present in any amount between 10% and 20%, although it isusual to have between 13% and 16% of this element. Cast irons containingthese high percentages of silicon are very hard and brittle.

The carbon content of such irons has to be much lower than is usual innormal cast irons, and will vary between 0.3% and 2.0% depending uponthe silicon content, and will tend to be lower the higher the siliconcontent. Almost all of the carbon in high silicon cast irons cast insand moulds exists as graphite. The amount and form of the graphiteconsiderably influences the mechanical properties of the material.Silicon reduces the amount of carbon required to form the eutecticconcentration, that is, the amount of carbonheld in solution in theliquid metal during the final'stages of solidification. For instance,for pure iron-silicon-carbon alloys the eutectic carbon concentration isabout 1.0% with about 13.5% silicon and only about 0.6% with 16.5%silicon. It is a matter of considerable practical importance (and alsoof difficulty) to keep the carbon content below the eutectic value. Thegraphite in sand-cast high-silicon cast irons usually exists in the formof coarse flakes, and graphite in excess of the eutectic concentrationin very coarse flakes. The presence of coarse flake graphite adverselyinfluences the mechanical properties of the metal. Also high-siliconcast iron containing coarse graphite tends to result in unsoundcastings, due to the formation of porosity cavities.

It is an object of the present invention to provide a high-silicon castiron in which coarse flake graphite is entirely eliminated. A furtherobject is to provide a high-silicon cast iron in which the graphite ispresent in very finely-divided form or as a mixture of veryfinely-divided graphite and nodular graphite. A still further object isto provide a process by which corrosionresistant cast irons free fromcoarse flake graphite can be obtained from irons which are hypoeutectic,eutectic or hyper-eutectic in carbon. Other objects will be apparentfrom the description which follows.

These objects are achieved according to the invention by theintroduction of not less than 0.02% by weight of cerium into the iron.

I have found that the addition of 0.02% to 1.0% by weight of cerium to amolten high-silicon cast iron immediately before pouring causes theeutectic carbon to separate on cooling in the form of veryfinely-divided undercooled graphite and any excess of carbon over theeutectic amount in the form of nodular graphite. The result is ahigh-silicon cast iron having exceptionally fine undercooled graphitestructures free from coarse flake graphite. The product shows improvedmechanical properties, including less tendency to brittleness, and. amarked freedom from porosity cavities.

In order that the cerium added shall exercise its full effect the ironmust be low in sulphur. Cerium is a powerful desulphurising agent, andif much sulphur is present cerium will be lost as cerium sulphide, whichfloats on the metal with the slag. I prefer to use an iron containingnot more than 0.2% y weight of sulphur, and the finished casting shouldcontain not more than 0.02% by weight of sulphur.

The cerium may be added in any convenient form, either as pure metalliccerium, mischmetall, ferro-cerium, cerium carbide or other alloy ofcerium. I prefer to avoid the use of cerium alloys containing magnesiumsince the latter appears to lessen the solubility of the cerium in theiron. The cerium may be applied in the form of a reducible ceriumcompound.

It will be seen that the addition of cerium in accordance with theinvention makes it possible to obtain satisfactory castings fromhigh-silicon irons with a much higher carbon content than has hithertobeen deemed acceptable. A further advantage is the reduction or completeelimination of the blowholes which, unless special precautions are takenin the melting and casting, may appear in high-silicon castings.

The invention is illustrated by the following example (in which allpercentages are by weight) A charge consisting of 18 lb. of pig-iron(8.94% total carbon, 2.78% silicon, 0.57% manganese, 0.017% sulphur, and0.029% phosphorus, the remainder being iron and adventitiousimpurities), 31 lb. of ferrosilicon (46% silicon) and 51 lb. of mildsteel scrap was melted in an indirect eleceans 3 tric arc furnace. 50lb. oi this metal was run into a ladle and 2 oz. oi. cerium mischmetallwas added before pouring the metal into a sand mould.

Analysis of a sample from the casting showed:

Microscopic examination of sections 01 the casting showed the graphitepartly in the form 01 very ilnely divided particles of undercooledgraphite and partly in the form of nodules or spherulites.

Having thus described my invention, .what I claim as new and desire tosecure by Letters Patcut is:

1. A cast iron 01 the corrosion-resistant type containing more than byweight of silicon and not less than 0.02% by weight of cerium.

2. A cast iron 01 the corrosion-resistant type containing more than 10%by weight of silicon, not more than the eutectic proportion of carbon,and not less than 0.02% by weight of cerium, the cast iron beingsubstantially free from coarse flake graphite.

3. A cast iron or the corrosion-resistant type containing more than 10%by weight of silicon and more than the eutectic proportion of carbon,characterised in that the carbon is present partly in the form ofnodular graphite. the rest of the carbon being in the form of finelydivided graphite.

4. A cast iron 01 the corrosion-resistant type containing more than 10%by weight of silicon and from 0.02% to 1.0% by weight of cerium.

5. A process for the production of corrosionresistant cast iron whichcomprises adding to a molten iron, containing more than 10% by weight oisilicon and not more than 0.2% by weight of sulphur, such an amount of acerium-providing a substance as will yield a casting containing not lessthan 0.02 71 by weight 0! cerium.

6. A process for the production 0! corrosionresistant cast iron whichcomprises adding to a molten iron containing more than 10% by weight ofsilicon, not more than the eutectic proportion 01 carbon, and not morethan 0.2% by weight of sulphur, such an amount of a cerium-providingsubstance as will yield a casting containing not less than 0.02% byweight of cerium.

'7. A process for the production of corrosionresistant cast iron whichcomprises adding to a molten iron. containing more than 10% by weight ofsilicon, more than the eutectic proportion 01 carbon, and not more than0.2% by weight of sulphur, such an amount or a cerium-providingsubstance as will yield a casting containing not less than 0.02% byweight of cerium.

8. A process as claimed in claim 5 in which the amount 01' cerium in thecasting does not exceed 1% by weight.

9. A process as claimed in claim 6 in which the amount 01' cerium in thecasting does not exceed 1% by weight.

10. A process as claimed in claim 7 in which the amount of cerium in thecasting does not exceed 1% by WBIEht.

BENTON MORROGH.

REFERENCES CITED The following references are of record in the file ofthis patent:

FOREIGN PATENTS Number Country Date 127,981 Great Britain June 11, 1919237,554 Great Britain Feb. 11, 1926 OTHER REFERENCES 40 Paper No. 875,presented at the 44th annual meeting of the Institute of BritishFoundrymen, June 17 to 20, 1947. Published by The Institute or BritishI'loundrymen, Manchester, England.

